A fuel cell of a polymer electrolyte fuel cell has a structure comprising: a membrane electrode assembly (MEA) which has been prepared by forming catalyst layers on respective surfaces of an electrolyte membrane having hydrogen ion conductivity; and gas diffusion layers (fuel-cell gas diffusion layers) stacked on the respective catalyst layers of the membrane electrode assembly. Reaction gases (fuel gas and oxidant gas) are respectively supplied to the gas diffusion layers, thereby generating electric power.
A catalyst layer is obtained by having carbon support platinum and then having the resultant product contain an electrolyte material (ionomer), and such catalyst layer constitutes an electrode layer that produces an electric-power generation reaction due to a supplied reaction gas. The catalyst layer formed on one of the surfaces of the membrane electrode assembly (MEA) constitutes an anode (fuel electrode), and the catalyst layer formed on the other surface thereof constitutes a cathode (air electrode).
A gas diffusion layer is a layer that is formed so as to enhance the diffusion property of a reaction gas, and such gas diffusion layer is arranged on a corresponding catalyst layer such that a reaction gas is uniformly supplied throughout the catalyst layer. Further, a gas diffusion layer has the function of discharging product water or humidified water from a catalyst layer and the function of effectively removing a current from a catalyst layer. In recent times, in order to further improve such functions of a gas diffusion layer, forming a conductive porous layer on a surface of a gas diffusion layer, which contacts a catalyst layer, has been employed.
This conductive porous layer is a porous layer that is made of a material having water repellency and conductivity and that is provided with an enormous number of fine pores. Thus, the function of discharging product water and humidified water is improved, and further, the function of removing a current from a catalyst layer is improved due to the increase of a contact area with the catalyst layer. A gas diffusion layer having such conductive porous layer is created, for example, by coating, onto one surface of a diffusion-layer base material consisting of carbon paper or carbon cloth, a pasty coating material prepared by mixing a conductive material and a water-repellent material and then baking the coated coating material via heating. The heated coating material then serves as a conductive porous layer.
A fuel cell is created by: respectively bringing conductive porous layers of gas diffusion layers into contact with catalyst layers of a membrane electrode assembly so as to achieve a state in which the membrane electrode assembly is sandwiched by the two gas diffusion layers; and subjecting the resultant product to joining via hot pressing, etc.
In light of the adhesion between a conductive porous layer and a catalyst layer, it is desirable for a conductive porous layer to have a lower porosity. On the other hand, in light of the gas diffusion of a conductive porous layer, it is desirable for a conductive porous layer to have a higher porosity. In order to satisfy such contradictory requests, Patent Document 1 below discloses a conductive porous layer having a two-layer structure in which a layer thereof on the catalyst layer side has a low porosity; and a layer thereof on the diffusion-layer base material side has a high porosity. With such configuration, the adhesion between a conductive porous layer and a catalyst layer can be improved while the gas diffusion throughout a conductive porous layer can also be improved.